kiji-express: EXP-275. Make the missing column values option for KijiInput more extensible.

issue: https://jira.kiji.org/browse/EXP-275
review: https://review.kiji.org/r/1339
commit: https://github.com/kijiproject/kiji-express/commit/d174aa5d521381d8a6782af936abb63144b0407b